interval: Add a pointer variant to compute_value()

ClutterInterval.compute_value() computes the new value given a progress
and copies it to a given GValue. Since most of the time we want to pass
that very same value to another function that copies it again, we should
have a compute_value() variant that stores that computed value inside
ClutterInterval and returns a pointer to it. This way we initialize the
result GValue just once and we never copy it, as long as the Interval
instance is valid.

clutter/clutter-interval.h

@@ -154,6 +154,9 @@ gboolean         clutter_interval_compute_value      (ClutterInterval *interval,
                                                       gdouble          factor,
                                                       GValue          *value);
 
+G_CONST_RETURN GValue *clutter_interval_compute      (ClutterInterval *interval,
+                                                      gdouble          factor);
+
 void clutter_interval_register_progress_func (GType               value_type,
                                               ClutterProgressFunc func);